Wheelbase Percentage

This has been on the back of my mind for a while but has been superceded by other motorhome stuff. I was going to reply on another thread but decided to start a new one.

My coach is 34' 9" long which I make as 417inches.
My wheel base is 216.5 which I make is 51.92%

Is that good or bad?

My question is, what is wheelbase percentage, and why does it matter? I never heard of it before.

Wheelbase to length ratio is one of the factors that determines how a vehicle handles.  An RV with a short wheelbase to length ratio will have a lot of wall area behind the rear axle.  When a gust of wind hits that wall, a lot of it's force will be behind the rear axle and will push the back of the vehicle sideways, rotating the front end in the opposite direction around the rear axle.  The driver has to countersteer against the rotation, then reverse direction when the gust stops.

If you have a longer wheelbase, there's less wall area behind the rear axle and side wind forces will be centered between the axles.  A gust of wind will still push the RV sideways, but the force pushes both the front and rear axles in the same direction.  The RV "leans" but doesn't need the same kind of countersteer and reversal as with a shorter wheelbase.

Same thing when a truck or other large vehicle overtakes the RV.  The pressure wave starts at the back of the RV and travels forward.  A short wheelbase RV will require more driver input to stay in a straight line than one with a longer wheelbase due to the greater leverage when the truck's bow wave hits the rear of the vehicle.

If you're towing a car, the effect is compounded.  When a long overhang vehicle turns, there is a lot of "tail swing".  The rear bumper first moves towards the outside of a turn, then is pulled through the turn.  A towed vehicle makes the same move, first away from the turn, then towards it.  If the RV wiggles back and forth when it by a gust of wind going down the highway, the toad is likewise pulled back and forth.

JD Gallant of the RV Consumer Group was the first person to publicize this effect back in the 1980s.  At that time some manufacturers were making extremely short wheelbase motorhomes, with wheelbase to length ratios less than 50% so there was a lot of wall area behind the rear axle.  The short wheelbase was easier to sell during a typical short, low speed test drives because the short wheelbase made the motorhome track around corners like a typical van or large car.  But they could be a real handfull at highway speeds compared to longer wheelbase units.

There are many factors that determine motorhome handling, but having a decent wheelbase to length ratio is one of the primary factors.  In general, less than 50% should be avoided, 50-55% is typical and anything over 60% is great.

Commercial buses make a close analogy..  Over the road buses have a long wheelbase for optimal handling at highway speeds.  School buses that spend most of their time at lower speeds and doing close maneuvering have short wheelbases and longer rear overhangs for more maneuverability.

Lou, I get the axle ratio issue with respect to gassers.  But does it apply equally to diesels?

The same theory holds, but diesels tend to be a lot heavier than gas RVs so side wind forces have less effect on the stability of the vehicle.

Also, diesels tend to have more front overhang, which shrinks the wheelbase to overall length ratio but doesn't really count as far as the wind affecting the handling.  It's the percentage of wall area behind the rear axle that has the greatest effect.

So a shade under 52% is OK but not great?

The wheelbase length percentage is one consideration.  There are others.  The most important being do you like the way it drives?  If not, then there are things you can do.  I strongly suggest you read Gary "RV Roamers" Brinks excellent in the library. 

I do like the way it drives. So much more after getting a front end alignment, fitting a road master reflex front stabiliser and dropping the tires to 85 psi (will drop them to 80 at the weekend).

Was just asking about the % out of curiosity. It's something I was not aware of when buying so will be on my radar if I change the coach in 2020 ;-)

Thanks, understand now. Had a 2014 Bounder 36R that had a lot of over hang in the rear. That MH scared the crap of of me most of the time. Going across Kansas, it was a handful. The handling was one of the things that made us trade it in on a DP. The Discovery 40X we have now handles like a sportscar compared to the 36R. 

52% is pretty decent for a gas chassis rig. Not that 55% wouldn't be better handling in the sense of greater stability, but the range for gas chassis A's is typically 50-53%.

Note that a longer wheelbase means that the turning circle is greater, so there are also advantages for a shorter one. No free lunch!

Sounds good to me Gary.

I do like the turning circle.

Bump.
Looking closely at 2012-2013 HR Ambassadors 36PFT.
Likes....
ok with chassis and motor
new enough for DW so it is updated.
Floorplan
Within budget
Have not driven yet.

Wheelbase to length ratio is 47.5% so this conversation scares me.
There is also a fair amount of HR bashing on the net.

Agenda is trip to the desert and back every year from snowy Michigan.

Shopping has been fun but I don't know what I don't know.
Please help with this topic. There will be more!
Thanks, Jeff

Yeah, that wheelbase is really short for a 37 footer. Best I can offer is to demand an extensive test drive in as much terrain as you can find and see how it handles. The Roadmaster RR8R chassis is general a solid performer with good road manners, so maybe the short wheelbase is of little importance. The 50% number was popularized by rv.org as a figure of merit, but it's not an absolute and there is no science behind it except longer is better.  Read this prior discussion of wheel bases:

http://www.rvforum.net/SMF_forum/index.php?topic=58588.0

I keep hearing this nonsense about the wind turning a coach with a long over hang. I have 13' of rear over hang and 20' in front of the rear axle. 
Wouldn't one think the wind would hit all 33' of my coach?
So why would the wind push the rear of my coach harder than it pushes the front?

It's not nonsense. Most of us who've had such coaches have actually experienced it, many times. If there were no resistance from the two axles, then your thoughts would be right, but you need to take that resistance into consideration. The axles do mimimize that effect for where they are located, but they are not evenly distributed along the length of the coach, thus the rear has less sideways resistance than the areas that have the axles to support them. In many ways, though, it's worse when a truck goes by, since the sidewash off that truck isn't hitting you evenly, rather it starts at the front, where the front axle minimizes the effect of the push, proceeds to the middle where the rear axle does the same, then finishes at the rear where there is nothing to stop that effect.

But let's use the exaggeration effect (often a great tool to help in understanding a principle), and change the configuration of the coach (assuming that there is still a reasonable weight balance) such that you have 6 feet in front of the rear axle and you have a 27 foot rear overhang. Does that help you see that a wind will push the rear around?

I don't know about the wind "turning" a coach with a long rear overhang, but certainly there is more leverage back there as the overhang gets longer. However much effect there is, a 13 foot overhang has more of it than  8-10 ft, right? A long lever always produces more force than a shorter one.

The biggest effect comes from passing semi's or quartering wind gusts, where the effect is not equal across the entire side of the rig. When another vehicle passes, it's "bow wave" of wind first strikes the rear of your rig and then progresses up the side to the front. You feel the steering pull one way as the wind hits the rear, then the other as the pressure moves up the side to the front.

Larry N. said:

It's not nonsense. Most of us who've had such coaches have actually experienced it, many times. If there were no resistance from the two axles, then your thoughts would be right, but you need to take that resistance into consideration. The axles do mimimize that effect for where they are located, but they are not evenly distributed along the length of the coach, thus the rear has less sideways resistance than the areas that have the axles to support them. In many ways, though, it's worse when a truck goes by, since the sidewash off that truck isn't hitting you evenly, rather it starts at the front, where the front axle minimizes the effect of the push, proceeds to the middle where the rear axle does the same, then finishes at the rear where there is nothing to stop that effect.

But let's use the exaggeration effect (often a great tool to help in understanding a principle), and change the configuration of the coach (assuming that there is still a reasonable weight balance) such that you have 6 feet in front of the rear axle and you have a 27 foot rear overhang. Does that help you see that a wind will push the rear around?

Click to expand...

Well with your 6'/27' exaggeration, you just prove my point. How can you even suggest the wind doesn't have as much effect in front of the rear axle as it does behind the rear of the axle. The wind will push evenly on all exposed surfaces. Geeshhhh.

Gary RV_Wizard said:

I don't know about the wind "turning" a coach with a long rear overhang, but certainly there is more leverage back there as the overhang gets longer. However much effect there is, a 13 foot overhang has more of it than  8-10 ft, right? A long lever always produces more force than a shorter one.

The biggest effect comes from passing semi's or quartering wind gusts, where the effect is not equal across the entire side of the rig. When another vehicle passes, it's "bow wave" of wind first strikes the rear of your rig and then progresses up the side to the front. You feel the steering pull one way as the wind hits the rear, then the other as the pressure moves up the side to the front.

Click to expand...

Gary, I agree with your comments about bow waves. But explain why I only feel the push right from the bow wave when passed by trucks, and then sucked back as they clear me. 33" Bounder F53, 13' over hang. No tale waggle at all.

I think it's just a matter of degree and even driver perception to some extent. The wheels/axles tend to keep the rig in place while the various pressures work to change it. The movable front wheels react to varying degrees. The use of a track bar (panhard rod) has an effect on that as well.

I'm not saying that a longer rear overhang makes the rig unmanageable. That was JD Gallant's (RV.org) contention, not mine.